1. Technical Field of the Invention
The present invention relates to an apparatus for recovering carrier, particularly relates to an apparatus for recovering decision-directed carrier of which baseband signal is processed in complex domain.
2. Description of the Prior Art
In a general demodulator of digital radio communication system, carrier recovery means the compensation of carrier frequency offset and phase offset contained in sample data in order to decide symbol data.
Carrier contained in a received signal is removed by a local oscillator in an intermediate frequency converter through a radio frequency receiving part. Even though the carrier is removed, there are the frequency offset and the phase offset of the carrier. Therefore, the frequency offset and the phase offset are detected and compensated in the baseband in order to simply implement demodulator.
In satellite communication, since high frequency is used for the intermediate and the radio frequency, the received signal is much affected by the frequency shift when bandwidth of transmission signal is narrow. When a narrow band signal such as voice signal is transmitted, the received signal is to be shifted by multiple numbers of the bandwidth. Therefore, there needs a frequency synthesizer which converts the received signal to an appropriate baseband signal.
Wide band satellite communication is not much affected by frequency shift. For example, in a satellite communication which transmits the signal having transmission rate more than 45 Mbps by Quadrature Phase Shift Keying (QPSK), if frequency shift is within 5000 PPM(Particles Per Million), the frequency shift may be under the control by using an accurate frequency oscillator.
Some methods have been proposed in order to improve carrier recovery performance.
In 1983, A. J. Viterbi and A. M. Viterbi proposed Mth Powering Algorithm (hereinafter, which is referred to xe2x80x9cVandV algorithmxe2x80x9d) for detecting carrier phase for MPSK (M-ary Phase Shift Keying) signal (See, A. J. Viterbi and A. M. Viterbi, xe2x80x98Nonlinear Estimation of PSK Modulation Carrier Phase with Application to Burst Digital Communicationxe2x80x99, IEEE Trans. Infor. Theory, vol. IT-32, July 1983). However, the VandV algorithm has a shortcoming that noise to signal considerably increases when M increases.
In 1991, Fitz analyzed vagueness of VandV algorithm and proposed solution (See, M. P. Fitz, xe2x80x98Equivocation in nonlinear digital carrier synchronizersxe2x80x99, IEEE Trans. On Comm., Vol. COM-39, No.11, November 1991).
Classen proposed a decision-directed method and the analyzed performance of the method (See, F. Classen, H. Meyer and P. Sehier, xe2x80x98An all feedforward synchronization unit for digital radioxe2x80x99, Proc. of VTC""93, 1993). In Classen""s method, though VCO (Voltage Controlled Oscillator) is not used, ROM is used in order to represent the detected carrier phase by complex value.
Also, Fitz proposed a decision-directed burst mode carrier synchronization techniques applicable to TDMA (See, M. P. Fitz, xe2x80x98Dicision-Directed Burst-Mode Carrier Synchronization Techniquesxe2x80x99, IEEE Trans. On Comm., Vol. COM-40, No.10, October 1992). This Fitz""s technique needs a divider in order to detect carrier frequency instead of VCO. Using the Fitz""s technique, frequency recovery may be obtained in broad range; however, it is difficult for high speed communication to use the divider, thereby being unsuitable to perform the frequency recovery.
Therefore, an object of the present invention is to provide a decision-directed carrier recovering apparatus which is apt for high rate satellite communication and rapidly recovers carrier phase and carrier frequency by processing baseband signals in complex domain.
According to the first aspect of the present invention, this object is accomplished by providing an apparatus for recovering carrier of an input signal from outside, the apparatus comprising: a) means for generating a first conjugate complex sample in accordance with a received complex sample; b) means for receiving the first conjugate complex sample and for recovering a carrier frequency of the first conjugate complex sample by compensating a carrier frequency offset of the input signal; c) means for receiving the first conjugate complex sample and for recovering a carrier phase of the input signal by compensating a carrier phase offset of the input signal; and d) means for selecting a symbol in accordance with output value from said means c) and for outputting the selected symbol to said means a).